Mass spectrometry, one of the most widely applied analytical techniques, has characteristics of high sensitivity, great accuracy, rapid analysis speed and strong qualitative capabilities. In order to meet the urgent demands of the on-the-spot real-time analysis and the online rapid detection and analysis, it is of great importance to develop miniaturized and portable mass spectrometers.
Traditional electro-spray ion sources are simple in structure, and their working process may be simply described as follows: a sample solution is made to flow through a capillary at a slow flow rate; the capillary is connected with a high voltage, and whether this voltage is positive or negative depends on the property of an analyte; the voltage provides an electric field gradient required by separation of charges on the liquid surface; under the action of the electric field, the liquid forms a “Taylor cone” at the capillary tip; when the solution at the tip of the Taylor cone reaches the Rayleigh limit, namely the critical point where the Coulomb repulsion of surface charges is equivalent to the surface tension of the solution, droplets containing large quantities of charges will be generated at the cone tip; with the evaporation of solvents, the droplets shrink, and repulsion among charges within the droplets increases; when this repulsion reaches and exceeds the Rayleigh limit, the droplets will undergo a Coulomb explosion to remove excess charges on their surfaces and to generate smaller charged droplets; the generated smaller charged droplets further undergo another explosion, and this process repeats again and again; eventually, gas-phase ions are obtained, and finally detected by a mass analyzer.
As for traditional electro-spray ion sources, gas-phase ions are generated in the atmospheric environment, which are then transmitted, via a sample introduction device, into the vacuum cavity where they are detected by the mass analyzer. In this process, losses of ions occur during their transmission, which restricts the signal intensity and detection limit of the analyte. Therefore, there is a wide application prospect of developing a simple vacuum electro-spray ion source that is featured by a simplified instrument structure and capable of reducing ion losses and improving the signal intensity and detection limit of a detected substance.